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Graphene Plasmon Study Open Doors for Optoelectronic Applications

KAIST researchers and their collaborators both at home and abroad have successfully demonstrated a brand new methodology for direct near-field optical imaging of acoustic graphene plasmon fields.

This strategy will provide a breakthrough for that practical applying acoustic graphene plasmon platforms in next-generation, high-performance, graphene-based optoelectronic devices with enhanced light-matter interactions and lower propagation loss.

It was recently demonstrated that graphene plasmons, collective oscillations of free electrons in graphene coupled to electromagnetic waves of light, can be used to trap and compress optical waves inside a very thin dielectric layer separating graphene from a metallic sheet.

In such a configuration, graphene’s conduction electrons are “reflected” in the metal, so when the light waves “push” the electrons in graphene, their image charges in metal also begin to oscillate. This new kind of collective electronic oscillation mode is called ‘acoustic graphene plasmon (AGP)”.

The existence of AGP could previously be viewed only via indirect methods for example far-field infrared spectroscopy and photocurrent mapping. This indirect observation was the cost that researchers needed to purchase the strong compression of optical waves inside nanometer-thin structures. It had been believed that the concentration of electromagnetic fields outside the device was insufficient for direct near-field optical imaging of AGP.

Challenged by these limitations, three research groups combined their efforts to bring together a unique experimental technique using advanced nanofabrication methods.

A KAIST research team led by Professor Min Seok Jang from the School of Electrical Engineering used a very sensitive scattering-type scanning near-field optical microscope (s-SNOM) to directly appraise the optical fields from the AGP waves propagating inside a nanometer-thin waveguide, visualizing thousand-fold compression of mid-infrared light the very first time.

Professor Jang and a post-doc researcher in the group, Sergey G. Menabde, successfully obtained direct images of AGP waves by taking advantage of their rapidly decaying yet always present electric field above graphene. They demonstrated that AGPs are detectable even when most of their energy is flowing within the dielectric underneath the graphene.

This became possible due to the ultra-smooth surfaces within the nano-waveguides where plasmonic waves can propagate at longer distances. The AGPmode probed through the researchers was up to 2.Three times more confined and exhibited single.4 times higher figure of merit in terms of the normalized propagation length when compared to graphene surface plasmon under similar conditions.

These ultra-smooth nanostructures from the waveguides utilized in the experiment are intended using a template-stripping method by Professor Sang-Hyun Oh along with a post-doc researcher, In-Ho Lee, in the Department of Electrical and Computer Engineering in the University of Minnesota.

Professor Young Hee Lee and his researchers at the Center for Integrated Nanostructure Physics (CINAP) of the Institute of Basic Science (IBS) at Sungkyunkwan University synthesized graphene with a monocrystalline structure, and this high-quality, large-area graphene enabled low-loss plasmonic propagation.

The chemical and physical properties of many important organic molecules could be detected and evaluated by their absorption signatures in the mid-infrared spectrum. However, conventional detection methods require a large number of molecules for successful detection, whereas the ultra-compressed AGP fields can offer strong light-matter interactions in the microscopic level, thus significantly improving the detection sensitivity down to just one molecule.

Furthermore, the research conducted by Professor Jang and the team indicated that the mid-infrared AGPs are inherently less sensitive to losses in graphene because of their fields being mostly confined inside the dielectric.

The research team’s reported results suggest that AGPs could become a promising platform for electrically tunable graphene-based optoelectronic devices that typically are afflicted by higher absorption rates in graphene for example metasurfaces, optical switches, photovoltaics, and other optoelectronic applications operating at infrared frequencies.

Professor Jang said, “Our research says the ultra-compressed electromagnetic fields of acoustic graphene plasmons can be directly accessed through near-field optical microscopy methods. I really hope this realization will motivate other researchers to use AGPs to various problems where strong light-matter interactions minimizing propagation loss are needed.”